As described in the article written by Souheng Wu of DuPont, USA (Polymer International, Vol. 29, No. 3, p 229-247 (1992)), plastics may be classified into pseudoductile plastics and brittle plastics. More particularly, plastics with a chain entanglement density (Ve) of less than 0.15 mmol/ml and a chain characteristic ratio (C) of larger than 7.5 belong to brittle plastics, which dissipate the external impact energy mainly by means of crazes from the matrix. The specific examples of brittle plastics are polystyrene, polymethyl methacrylate, acrylonitrile-styrene resins and the like. On the other hand, plastics with a chain entanglement density (Ve) of not less than 0.15 mmol/ml and a chain characteristic ratio (C) of not more than 7.5 belong to pseudoductile plastics, which dissipate the external impact energy mainly by generating the shear yielding from the matrix. With respect to the plastics between pseudoductile ones and brittle ones and pseudoductile plastics having a chain entanglement density (Ve) of about 0.15 mmol/ml, the present inventors define them in the specification as “less brittle plastics”. The specific examples of less brittle plastics are polyvinyl chloride, polypropylene, polyformaldehyde and the like.
It is well known in the prior art that brittle or less brittle plastics can be toughened by incorporating a rubber into the polymerization system during the preparation of the plastics. For example, high-impact polystyrene toughened by rubber particles having a “salami” structure can be prepared by dissolving a cis-1,4-polybutadiene rubber in styrene monomer and then polymerizing said monomer. However, such a method is relatively complicated and is not necessarily suitable for other types of brittle plastics. Another method for toughening brittle or less brittle plastics involves melt-blending rubbers with plastics, i.e. toughening brittle or less brittle plastics by incorporating unvulcanized rubbers or thermoplastic elastomers. The specific example of such a method is to toughen polypropylene with ethylene-propylene rubbers. In such a method, it is very difficult to effectively control the dispersing state of the rubbers in the plastic matrix, since the morphology of the rubber particles in the plastic matrix and their distribution largely affect the toughening effect, which renders the toughening effect undesirable. Moreover, more rubbers are normally needed to be incorporated so as to achieve a desirable toughening effect, which will largely deteriorate the properties of the plastics, such as strength, stiffness and the like.